1 Field of the Invention
The present invention relates to a method for forming a patterned oxide superconductor thin film, more specifically to a method for forming a patterned oxide superconductor thin film without degrading the oxide superconductor thin film itself.
2. Description of Related Art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature. Researches on Josephson junction devices, superconducting transistors, superconducting field effect devices, etc. utilizing those oxide superconductors are now in progress.
A Josephson junction device which is one of well known superconducting devices can be realized in various structures. Among the various structures, the most preferable structure in practice is a stacked junction realized by a thin non-superconductor layer sandwiched between a pair of superconductors. However, a point contact type junction, a micro bridge type junction and a variable thickness bridge type junction which are composed of a pair of superconductor regions which are weakly linked to each other also exhibit Josephson effect.
Since these Josephson junctions have fine structures which are in order of a few times of the coherent length, it is difficult to obtain Josephson junction devices having homogeneous and stable properties. In particular, oxide superconductors are easily degraded during processings so that it is difficult to manufacture Josephson junction devices of the above types utilizing oxide superconductors having excellent properties.
In order to resolve the above problems, there is proposed a different type junction which can be manufactured by using oxide superconductors without fine processings. This type of a Josephson junction is so called a grain boundary type Josephson junction which consists of two superconducting electrodes formed of single crystal films of an oxide superconductor having different crystalline orientations and a grain boundary between them. In this type of the Josephson junction, the two superconducting electrodes are considered to be weakly linked through the grain boundary so that the grain boundary forms a weak link of a Josephson junction.
This type of the Josephson junction is manufactured by depositing an oxide superconductor thin film on a single crystalline substrate having a trench on its deposition surface under a condition suitable for growing a single crystalline oxide superconductor thin film. A grain boundary is automatically formed in the oxide superconductor thin film at the trench portion. Since, a different lattice plane is exposed at the trench portion of the substrate which have an effect on a crystal orientation of the oxide superconductor thin film growing at the trench portion. For this purpose, the step should be sharply formed.
Though the Josephson junction device is one of the most famous superconducting devices, logic circuits may be more easily assembled by using so called superconducting-base transistors or so called super-FETs (field effect transistor) which are three-terminal superconducting devices than by using Josephson junction devices which are two-terminal superconducting devices. Therefore, the superconducting-base transistor and the super-FET are more practical.
These superconducting devices have superconducting parts as superconducting electrodes, superconducting channels etc. These superconducting parts are usually formed of superconducting thin films.
To apply superconducting devices to various electronic equipments, these superconducting devices need to be incorporated within an IC (integrated circuit). There are many element devices on a substrate of an IC and each element device region is isolated. A superconducting thin film which has an isolated superconducting region is necessary to isolate a superconducting device region which is incorporated within IC. In case of a superconducting device formed of an oxide superconductor material which has been recently advanced in study, an oxide superconductor thin film which has an isolated superconducting region is necessary.
Researches are also advanced in applications of oxide superconductors to microwave circuits. In general, a microstrip line used in the microwave circuit has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to the frequency. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly improve the high frequency characteristics of the microstrip line. Namely, by using a superconducting microstrip line, the loss can be significantly decreased and microwaves of higher frequency range can be transmitted. In particular, the loss is lower in the oxide superconducting microstrip line than in the normal microstrip line utilizing Cu for the conductor at a frequency of up to about 100 GHz.
Additionally, microwaves are characterized by a straight-going property of radio waves and small diffraction due to their short wave length of a few millimeters to several tens centimeters. Therefore, special and unique methods and devices are needed for handling microwaves.
As regards impedance elements such as L, C, R, suitable ones for microwave circuits have different shapes from those of ordinary electronic circuits. Discrete elements which usually become distributed elements in the microwave circuits are not suitable due to their properties and large size, which becomes an obstacle for manufacturing high density integrated microwave circuits. In addition, the discrete elements can not prepared simultaneously with microwave circuits, so that the discrete elements are attached to the microwave circuits afterwards. In this case, microwave reflection would occur at a connection between a discrete element and a circuit due to discontinuity of transmission line, so that properties of the microwave circuit become lowered.
On the other hand, lumped elements have smaller dimensions and can be formed as a part of microstrip lines. Therefore, it is possible to prepare lumped elements simultaneously with microwave circuit so that monolithic microwave integrated circuits can be obtained.
Since the above lumped elements are prepared by processing microstrip lines, it can be clearly understood that oxide superconductors are easily applied to the above lumped elements, which significantly improve their properties than conventional materials than conventional materials.
In a prior art, in order to prepare the oxide superconducting devices, a patterned thin film of a material including silicon, for example a patterned SiO.sub.2 thin film is at first formed on a substrate. Then, an oxide superconductor thin film is formed over the substrate. Silicon diffuses into the oxide superconductor thin film on the SiO.sub.2 thin film so that the oxide superconductor thin film there becomes nonsuperconducting. The other portion of the oxide superconducting thin film growing directly on the substrate becomes superconducting, so that a superconducting device consisting of the patterned oxide superconductor thin film is completed.
In the above methods the SiO.sub.2 thin film is mostly patterned by a lift-off process. Namely, a photoresist layer is formed on a portion of the substrate and the SiO.sub.2 thin film is deposited over the substrate and on the photoresist. Then, the photoresist layer is removed with a portion of SiO.sub.2 thin film on the photoresist layer. The substrate is disposed at the portion at which the photoresist layer is removed.
However, in the above prior art the substrate is reacted with the photoresist and/or photoresist: remover so that defects and residual spots are generated at the surface of the substrate. An oxide superconductor thin film formed on these imperfect surfaces of the substrates have worse superconducting properties than expected, so that the superconducting devices do not show enough performances.